[base-ui] Fix mergeSlotProps className join order

[mj12albert]

Fixes #39601

Before: https://codesandbox.io/s/merge-slot-props-classes-order-w9tkzp?file=/src/App.tsx

After: https://codesandbox.io/s/https-github-com-mui-material-ui-pull-39616-syxp9x?file=/src/App.tsx

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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[mj12albert]

After #39032 (comment) I have been leaving the className prop in externalForwardedProps (in FormControl, FilledInput) @DiegoAndai

It turns outs if className is explicitly passed to the className parameter of mergeSlotProps (AND if getSlotProps is not passed), it hides the problem that the className merge order is wrong, since className overrides externalSlotProps?.className

Luckily InputLabel contained this test and it crossed my path 😅 otherwise the issue could have stayed hidden for a lot longer

[DiegoAndai]

I agree with these changes 👍🏼 there should be consistency.

I won't approve yet because I don't have the context about which order is correct, so I would wait for either @mnajdova or @michaldudak to review.

[michaldudak]

Nice catch and fix! Implementing Material UI components and testing them with Base UI primitives is the best thing that could happen for this library.